In a medium bowl or bowl of a stand mixer, mix together cake flour, sugar, baking powder, baking soda, and salt on low speed for about 20 seconds. Add softened butter and oil and mix for about 1 minute to coat the flour, until mixture resembles wet sand.

In a small bowl, whisk together the egg, yolks, vanilla, butter flavor and sour cream. Slowly pour in 2/3 of the wet mixture and beat on medium speed for about 1-2 minutes, until smooth. Scrape down bottom and sides of bowl one last time to make sure everything is fully combined. Beat in remaining wet mixture and beat for about 30 more seconds until fully incorporated.

Distribute the batter between the prepared cake pans. Bake 30 to 35 minutes*, or until a toothpick inserted in the center comes out clean. Cool slightly in pans for about 5 minutes, before transferring to a wire rack to cool completely. Let cool before frosting.

Frosting:

With an electric mixer, beat the softened butter on medium-high speed until light and creamy, about 3-5 minutes. Beat in vanilla and salt. Add the powdered sugar, 1 cup at a time on low speed until fully incorporated. Beat in corn syrup and 2-4 tablespoons of cream, until smooth. Sift in cocoa powder. Add in remaining cream, 1 tablespoon at a time, until desired consistency is reached. Scrape down bottom and sides of bowl. Increase to medium-high speed and beat for an additional 3-5 minutes until smooth, light and fluffy.

Frost completely cooled cakes. Store in an air tight container at room temperature.

*Bake 25-30 minutes for two 9-inch rounds, 40-45 minutes for two 6-inch rounds, or 30-35 minutes for a 9x13-inch pan